Hereinafter, CCD solid-state imaging devices in the related art will be described with reference to the drawings.
Recently, in solid-state imaging devices, the number of pixels in the horizontal direction has been increased in order to meet a demand for higher resolution. This results in a higher driving rate of a horizontal CCD register (horizontal transfer unit), causing problems such as reduction in transfer efficiency and increase in electricity consumption. As a method for solving such problems, techniques for providing a plurality of horizontal transfer units arranged in parallel have been reported (for example, see Patent Literatures 1 and 2).
In the solid-state imaging device described in Patent Literature 1, the shape of a storage electrode in a horizontal transfer unit for performing branch transfer is devised, or the concentration of impurities on the branch transfer unit side (in the vertical transfer direction) is increased in the horizontal transfer unit for performing branch transfer. Thereby, a potential gradient is formed in the vertical transfer direction to suppress branch transfer failure. Unfortunately, in these structures, the potential gradient is always formed under a horizontal electrode. This causes various problems: the saturated charge amount in the horizontal transfer unit is reduced; or signal charges are horizontally transferred in a meandering manner during horizontal transfer to increase an effective transfer length per transfer stage, resulting in transfer failure.
In order to improve such problems, the solid-state imaging device described in Patent Literature 2 includes a storage electrode in a horizontal transfer unit for performing branch transfer wherein the storage electrode includes a plurality of electrode elements, and charges are transferred by applying a different driving pulse to each of the electrode elements during branch transfer.
FIG. 19 is a configuration diagram showing part of a horizontal transfer unit of a solid-state imaging device in the related art, which is described in Patent Literature 2. FIG. 20 is a potential schematic view showing a cross-sectional structure of a topmost layer of the horizontal transfer unit taken along the line B-B′ shown in FIG. 19, and potential distributions and transfer states of signal charges under the respective electrodes. FIG. 21 is a drawing showing a timing at which the respective transfer pulses are applied to the corresponding electrodes in FIG. 19 during a horizontal blanking period. FIG. 20 shows the potential distributions and transfer states of signal charges under the respective electrodes during Periods N1 to N5 in FIG. 21.
The solid-state imaging device described in Patent Literature 2 includes a channel stop 201, an intermediate transfer unit 206, a final vertical transfer electrode 207, horizontal storage electrodes 209 and 211, a horizontal barrier electrode 210, and a barrier region 213 as shown in FIG. 19. The horizontal storage electrodes 209 and 211 and the horizontal barrier electrode 210 form a horizontal transfer electrode. The horizontal transfer electrode has a structure for horizontal two-phase drive in which voltages φH1 and φH2A (φH2B) are applied to the corresponding electrodes during the horizontal transfer to transfer the signal charges via a channel 215 provided in a semiconductor substrate 214 shown in FIG. 20.
The horizontal storage electrode 211 transfers the signal charges from a first horizontal transfer unit 204 to a second horizontal transfer unit 205, and includes a first electrode element 211A and a second electrode element 211B. The first electrode element 211A covers substantially the center of the channel in the first horizontal transfer unit 204, the intermediate transfer unit 206, and the channel in the second horizontal transfer unit 205, and forms a storage electrode for the first horizontal transfer unit 204 and the second horizontal transfer unit 205. The second electrode element 211B covers the channel in the first horizontal transfer unit 204 not covered with the first electrode element 211A, and forms a storage electrode for the first horizontal transfer unit 204. The first electrode element 211A is electrically insulated from the second electrode element 211B. The second electrode element 211B has approximately the same width as that of the first electrode element 211A. As shown in FIG. 19, when observed from above, the second electrode element 211B is overlaid on the first electrode element 211A in a region ranging from the second horizontal transfer unit 205 to approximately a half region on a branch transfer electrode 208 in the first horizontal transfer unit 204.
The storage electrode for the second horizontal transfer unit 205 is composed of only the first electrode element 211A, while the storage electrode for the first horizontal transfer unit 204 is composed of the second electrode element 211B covering approximately a half of the first horizontal transfer unit 204 on the side of the final vertical transfer electrode 207 and the first electrode element 211A covering approximately a half of first horizontal transfer unit 204 on the side of the intermediate transfer unit 206.
The second electrode element 211B and the horizontal barrier electrode 210 are commonly connected while the first electrode element 211A is independently connected to an outside.
Hereinafter, transfer operation will be described using FIGS. 19 to 21. First, φH1 to be applied to the horizontal storage electrode 209 and φH2A and φH2B to be applied to the horizontal storage electrode 211 (first electrode element 211A, second electrode element 211B) are simultaneously set to a high level, and φVL to be applied to the final vertical transfer electrode 207 is set to a low level. Thereby, the signal charges accumulated under the final vertical transfer electrode 207 in a vertical transfer unit 202 (solid dot shown in FIG. 19) are transferred to under the horizontal storage electrode 209 and the horizontal storage electrode 211 (first electrode element 211A, second electrode element 211B) corresponding to the first horizontal transfer unit 204 (Period N1, FIG. 20(a)).
Next, φT to be applied to the branch transfer electrode 208 is set to the high level, φH1 and φH2B are set to the low level at the same time, and φH2A is set to a middle level between the high level and the low level. Thereby, the signal charges are transferred to the intermediate transfer unit 206 (Period N2, FIG. 20(b)).
Subsequently, φH2A is set to the low level, and transfer of the signal charges to the branching channel is completed (Period N3 FIG. 20(c)).
Next, φH1 is set to the high level, and φT is set to the low level. Thereby, the signal charges in the intermediate transfer unit 206 are transferred to under the horizontal storage electrode 209 in the second horizontal transfer unit 205 (Period N4, FIG. 20(d)).
Meanwhile, the signal charges transferred from the vertical transfer unit 203 (blank dot in FIG. 19) remain within the first horizontal transfer unit 204 during Periods N2 to N4. Thereby, the signal charges in the vertical transfer units 202 and 203 are branched to the first horizontal transfer unit 204 and the second horizontal transfer unit 205, and branch transfer is completed.
Subsequently, one branch of the signal charges is horizontally transferred within the first horizontal transfer unit 204 to an output unit (not shown), and the other branch thereof is horizontally transferred within the second horizontal transfer unit 205 to an output unit (not shown).